1. Field of the Invention
The present invention relates to a method for manufacturing a flash memory device and specifically, to a method for manufacturing a flash memory device, which improves an operational characteristic of a memory cell by preventing the effect that a tunnel oxide film is formed thinner at top corners of a trench than in an active region while forming a field isolation film with a conventional shallow trench isolation (STI) process.
2. Discussion of Related Art
In fabricating a flash memory device, it is general to form a field isolation film by means of a self aligned shallow trench isolation (SA-STI) process. But, with the trends of higher integration density for flash memory devices, the technique of SA-STI becomes no more effective under the dimension of sub micron scheme. That is, it would be inevitable, under the sub micron dimension, to employ a typical shallow trench isolation (STI) technique that conducts an etching process (hereinafter, referred to as ‘trench etching process’) to form trenches in the structure including oxide and nitride films.
However, since the STI process forms a tunnel oxide film after forming a trench in first, there is a problem that a profile of the tunnel oxide film is affected from a profile of top corners of the trench. In general, as shown in FIG. 12, a part of the tunnel oxide film is formed thinner at the top corners of the trench, rather than the other part of the tunnel oxide film in an active region, because an oxidation rate is reduced due to plasma damages during an oxidation process to form the tunnel oxide film. As a result, it becomes difficult to accomplish a normal cell operation.